In such tractor-trailer combinations, the connecting point of the trailer vehicle to the tractor vehicle is fixed and unalterable. The ball-headed coupling part must thus project a not inconsiderable extent beyond the rear of the tractor vehicle to substantially facilitate the coupling and uncoupling of the trailer vehicle, but this causes problems as regards the handling properties of the tractor-trailer combination. Not only the trailer vehicle load, but also the dynamic forces arising during movement, act at a point on the tractor vehicle which is a considerable distance from the center of gravity and is far behind the rear axle of the tractor vehicle.
Although the handling properties of such tractor-trailer combinations have been substantially improved by altering the constructions of the trailer vehicle axles, for example by designing them as tilted shaft axles, the problem of the zig-zag and side-to-side movement of the trailer vehicle, particularly when travelling at high speed in the forward direction, has not hitherto been satisfactorily solved, and certainly not in an optimum manner. No surprising solution to this problem was expected because developments over the last few decades have always involved joining the trailer vehicle to a coupling point disposed outside the outer perimeter of the tractor vehicle. The zig-zag and side-to-side movement of the trailer vehicle (by this is meant an oscillating yawing movement of the trailer about a vertical axis), which often occurs when travelling over uneven surfaces and particularly at high speeds, acts on the tractor vehicle via the one coupling point and causes it to execute a counter-movement, which although absorbed and damped by the four wheels and two axles of the tractor vehicle, nevertheless adversely affects the handling behavior of the tractor-trailer combination.